Determining fluid composition from fluid properties

Abstract

A method for determining the composition of a fluid by using measured properties of the fluid. One embodiment of the method of the current invention generally comprises: selecting a fluid property that has a response to fluid composition that is linear, or can be approximated as linear; measuring the selected fluid property at a series of specific time intervals; and plotting the measured property as a function of the selected property. In effect, plotting the measured property as a function of fluid composition in an arbitrary set of units. This allows for a in-situ qualitative evaluation of fluid composition by measuring a fluid property that has a known linear relationship to fluid composition. Another embodiment of the present invention further comprises, establishing the endpoints of contamination and plotting the measured properties through these endpoints. Once the endpoints have been established a quantitative evaluation of the fluid composition can be performed. One preferred fluid property that can be used is density.

1. A method for determining a composition of a downhole fluid mixture comprising:
 measuring at least one property of the fluid mixture; 
 determining a normalized fluid contamination using a measured fluid property and the response of the measured fluid property to fluid contamination, the response being determined from a mixing law; 
 presenting the measured property versus the normalized fluid contamination; and 
 determining the composition of the downhole fluid mixture based on the relation between the measured property and the normalized fluid contamination. 
 
   
   
     2. The method of  claim 1  further comprising:
 determining a value of the property of the first component fluid; 
 determining a value of the property of the second component fluid; and 
 using the values of the fluid properties of the component fluids and appropriate mixing law to determine the relative fractions of each of the components in a mixture. 
 
   
   
     3. The method of  claim 1  wherein the measured fluid property is density. 
   
   
     4. The method of  claim 2  wherein the measured fluid property is density. 
   
   
     5. A method for determining a composition of a downhole fluid mixture comprising a first component and a second component, comprising:
 measuring a density of the downhole fluid mixture; 
 determining a normalized fluid contamination using the measured fluid density and the response of the measured density to fluid contamination, the response being determined from a mixing law; 
 plotting the measured density versus the normalized fluid contamination; and 
 determining the composition of the downhole fluid mixture based on the relation between the measured property and the normalized fluid contamination. 
 
   
   
     6. The method of  claim 5  further comprising:
 determining a density of the first component of the downhole fluid; 
 determining a density of the second component of the downhole fluid; and 
 using the densities of the first and second components of the downhole fluid as end points of a curve created by plotting the measured density versus the actual fluid contamination. 
 
   
   
     7. The method of  claim 5  wherein the normalized fluid contamination is calculated using the response of density to fluid contamination, wherein the response is expressible as 
     
       
         
           
             
               
                 
                   ⅆ 
                   
                     ρ 
                     mix 
                   
                 
                 
                   ⅆ 
                   f 
                 
               
               = 
               
                 ( 
                 
                   
                     ρ 
                     1 
                   
                   - 
                   
                     ρ 
                     2 
                   
                 
                 ) 
               
             
             , 
           
         
       
     
     wherein ρ 1  is the density of a first component of the wellbore fluid, ρ 2  is the density of a second component of the wellbore fluid and, 
     
       
         
           
             
               ⅆ 
               
                 ρ 
                 mix 
               
             
             
               ⅆ 
               f 
             
           
         
       
     
     is the incremental change of the density of the wellbore fluid, ρ mix , to the change in fluid contamination, f. 
   
   
     8. A method for determining a composition of a downhole fluid mixture comprising:
 pumping the downhole fluid mixture through a downhole measurement module; 
 measuring at least one property of the fluid mixture that has a response to fluid composition being determined from a mixing law; 
 transmitting the measured fluid property data to a control system at the surface; 
 determining a normalized fluid contamination using the measured fluid property and the response of the measured fluid property to fluid contamination; 
 plotting the measured density versus the normalized fluid contamination; and 
 determining the composition of the downhole fluid mixture based on the relation between the measured property and the normalized fluid contamination. 
 
   
   
     9. The method of  claim 8  further comprising:
 determining a maximum value of the fluid property; 
 determining a minimum value of the fluid property; and 
 using the maximum and minimum values of the fluid property as end points of a curve created by plotting the measured property versus the actual fluid contamination. 
 
   
   
     10. The method of  claim 1  further comprising presenting the measured property versus the normalized fluid contamination in a graphical display. 
   
   
     11. The method of  claim 5  further comprising plotting the measured density versus the normalized fluid contamination on a graphical display. 
   
   
     12. The method of  claim 8  further comprising plotting the measured property versus the normalized fluid contamination on a graphical display.